A known spur gear transmission of EP-A-686788 has a single-piece housing. The latter forms, between a drive-output-side housing wall and a partition, a drive-output-side chamber which contains the drive output shaft with the drive output gearwheel. In order that the drive output gearwheel can be mounted in the drive-output-side chamber, its side wall has to have an assembly opening. This is achieved in that the drive-input-side housing cover is connected to the transmission housing by means of a parting joint which runs at an incline and extends above the drive-output-side chamber. The cover is connected to the transmission housing by fastening screws which run in an axially parallel manner, that is to say not perpendicularly to the parting joint. The housing also forms a drive-input-side chamber into which the motor pinion projects through the cover. Said motor pinion meshes with an intermediate gearwheel which is arranged on an intermediate shaft in the drive-input-side chamber. The intermediate shaft supports an intermediate pinion which drives the drive output gearwheel. Said intermediate shaft is mounted at both sides of the intermediate pinion, specifically in the drive-output-side wall and in the partition. The drive-input-side, exposed end of the intermediate shaft projects into a corresponding bore of the cover in order to centre the latter. This bore is not designed as a bearing.
Said known arrangement has not been proven. Firstly, the assembly is very difficult because the fastening screws exert forces on the cover, which forces have a force component running parallel to the parting joint, which force component seeks to move the cover relative to the housing. However, a movement of said type absolutely must be avoided because it leads to friction contact of the shaft with the centering bore in the cover. This can not only result in damage to the shaft, but can also lead to overloading of the intermediate shaft bearing. It is not obvious how this problem could be solved. In particular, the inclined parting joint cannot easily be replaced with the otherwise customary parting joint which runs perpendicularly to the axial direction because then the assembly of the drive output gearwheel would not be possible. A further disadvantage of the known construction consists in that the inclination of the parting joint requires precise adjustment of the cover about an axis perpendicular to the inclined parting joint, since an imprecise positioning of the cover with respect to this axis would lead to an angular error in the toothing of the input drive pinion with the intermediate gearwheel of the first stage. It is barely practically possible to obtain said precise adjustment. Finally, the known transmission has the disadvantage that the stiffness of the housing is impaired by the inclined profile of the parting joint which extends into the drive-output-side chamber.
Given that the drive output shaft can be mounted with the drive output gearwheel and the associated bearings in the housing which is designed without an undercut, the creation of a lateral assembly opening by an inclined cover joint is unnecessary. The disadvantages explained above can thus be avoided. The assembly is very simple because after inserting the shafts and wheels it is necessary merely to put on the cover. As a result of the third bearing, which is provided in the cover, of the intermediate shaft, the cover is automatically centered. In contrast, the triple bearing arrangement leads to favourable force conditions at the intermediate shaft and its bearings, thereby permitting smaller dimensioning of said bearings and therefore a reduction of the spatial requirement. There are also no additional assembly steps required for axially fixing the shaft, because said axial fixing can be effected by means of the two bearings at the ends of the shaft.
Although a triple bearing arrangement of a transmission shaft is known (“SEW Eurodrive” brochure, FIG. 5.3), this relates to a transmission having a multi-part housing which has an assembly opening not only at the drive input side but also at the drive output side of a housing central part and contains a partition. The triple bearing arrangement of the intermediate shaft of said transmission is situated in bearing bores which are formed by the housing central part. This entails cumbersome assembly, because the intermediate shaft and its bearings must be mounted from both sides of the housing central part. In addition, the multi-part design of the housing has the disadvantages, in relation to single-part housings having only one assembly opening, of reduced stiffness or, for the same stiffness, increased weight and increased costs. It is not possible to gather any indications from said prior art of how to facilitate assembly and avoid centering errors in the transmission specified at the beginning.